From: Amara Graps (amara@amara.com)
Date: Thu Oct 21 1999 - 14:28:16 MDT
Robert J. Bradbury (bradbury@www.aeiveos.com) Wed, 20 Oct 1999 writes:
>I find it just too convenient that big stars like to spit
>out lots of carbon and carbon just "happens" to be both
>good for building wet nanotech *and* dry nanotech. Why
>isn't the hardest material "boron" or something that is
>*really* scarce? One could argue that common universes
>could be structured to allow the evolution of wet nanotech
>but dry nanotech would be very resource limited! Instead
>we have a playing field that seems optimal for both random
>and directed evolution of complexity.
About the occurrence of carbon in big stars.
Robert, I'm not sure if your question is:
Why carbon ?
(The answer is "because nucleosynthesis says it must be")
or
Why is nucleosynthesis like this in our universe ?
(I don't know the answer to that)
I'll answer to the first one. Perhaps you know about nucleosynthesis
and stellar evolution, in which case, you can pass this message
over, but some others here may not know these things, so I'll write
a little about it.
To begin, I'll mention that the theory of stellar structure and
evolution is probably the most theoretically and experimentally
sound aspect of astrophysics and it has a critical role in our
current understanding of the universe (stellar ages etc.).
In ordinary main-sequence stars like the Sun, the primary process
for fusion is the proton-proton chain. In this reaction, four protons
(these are the hydrogen nuclei) are fused into one helium nucleus
(also called an alpha particle). An alpha particle has about 1% less
mass than four protons. The rest of the mass is turned into gamma
rays - high energy photons.
In chemical notation:
4 1H + 2 e --> 4He + 2 neutrinos + 6 photons
Fusion can only happen when the protons can get close to each other.
It's a tricky business since the protons both have a positive
charge, and at lower temperatures they repel each other, so then one
needs to have high temperatures, like those inside of a star, so
that the protons can hit each other at high speed. The proton-proton
chain has several intermediate steps where electrons and
antielectrons annihilate each other, more protons collide into each
other, and a helium atom is made and with the helium and neutrons
and protons bouncing around in the hot gas.
In chemical notation, it looks like:
1H + 1H --> 2H + antielectron + neutrino
1H + 1H --> 2H + antielectron + neutrino
electron + antielectron --> photon + photon
electron + antielectron --> photon + photon
2H + 1H --> 3He + photon
2H + 1H --> 3He + photon
3He + 3He --> 4He + 1H+ 1H
Or Rewriting:
6 1H + 2 e --> 4He + 2 1H + 2 neutrinos + 6 photons
Which is just our original equation above:
4 1H + 2 e --> 4He + 2 neutrinos + 6 photons
So now we have made helium. What's next?
After the exhaustion of hydrogen, the next reactions that may take
place in the center of the star involve 4He to fuse into carbon.
Helium burning has to take place through a nuclear process called
the triple-alpha process;
3 4He --> 12C + photon
A nucleus could continue to grow by successively capturing alpha
particles, and, indeed, this is the initial sequence of
nucleosynthesis in heavier and heavier stars.
12C + 4He --> 16O
16O + 4He --> 20Ne
20Ne + 4He --> 24Mg
.
.
.
Now, why will smaller, ordinary stars like our Sun not create heavier
elements than helium?
Heavier elements generally have larger electric charges. To fuse
them into yet heavier elements requires overcoming greater Coulomb
barriers than for elements with small electric charges; so such
reactions will not be initiated until the star acquires higher
temperatures (the same reasoning already discussed in the
proton-proton chain introduction) Thus, helium reactions generally
require higher temperatures than hydrogen reactions; carbon
reactions, higher temperatures than helium reactions, etc. This
pattern is a general feature of thermonuclear reactions of charged
nuclei inside stars. The more massive a star, the hotter (and
faster, lifetimes of massive stars are shorter) it burns, and the
heavier the elements that it generates inside it.
A star spends most of its time burning hydrogen into helium, and
this evolution time is called the "main sequence". After hydrogen is
exhausted near the star's center, the star is left with a core
consisting of helium and a small amount of heavy elements.
Initially, the temperature of the core is below the E8 K required
for helium ignition. But as the star contracts through gravitational
contraction, the center can burn hot enough to ignite helium.
Our Sun won't burn hot enough during its evolution to fuse helium
into carbon. But heavier stars will. When helium is exhausted,
the next element to react is 12C which takes place temperatures
around 5-10 E8 K.
So you see, an important feature in understanding nucleosynthesis is
the energetics of fusion, as determined by the atomic mass excesses
and the mean binding energy per nucleon. "boron" doesn't have the
right atomic and binding energy to fit in the scheme of the
nucleosynthesis processes (P-P chain, CNO chain), and so you see how
carbon is a natural fusion product of heavier stars.
Now, a side note about Iron and those really heavy elements
that you find in terrestrial planets and sometimes in humans :-)
The structure of the nucleus of and atom can be understood roughly
as a balance between the attractive and repulsive parts of the
strong nuclear force between protons and neutrons, modified by the
electric repulsion between the protons. The strong nuclear force has
a kind of attractive-repulsive duality. As long as the nucleus is
not too big, the nuclear force, being short-range, tends to win over
the repulsion of the electric force. It is then advantageous to add
protons or neutrons to have heavier nuclei. But only if the nuclei
can keep together- "binding".
But at Iron-56, the nuclear binding energy saturates - its nucleus
is the largest in which the constructive short-range strong force
has any advantage over the destructive long-range electric force. If
one adds more protons or neutrons, then the binding energy per added
particle decreses, and the nuclei becomes unstable and starts
decaying or starts fissioning (uncontrolled fission chain reactions
led to the A-bomb, for example). And radioactive decay, is a useful
way to date rocks because unstable nuclei doesn't last for long, and
so the existence of elements like uranium and thorium in something
can give us that object's age.
So if you want to make elements heavier than iron, then you have to
look to unusual processes to overcome all of the above physical
issues. In nucleosynthesis, two processes that can make elements
heavier than iron is the "s-process" and the "r-process" and the
"p-process". These processes only can occur in really massive stars,
where the conditions inside are extreme.
Then you need a way to push those elements out into space. An
explosion will do it. The dust that you see in supernovae remnants
was mostly _not_ produced in the supernovae event. It was present
in the star's outer envelope before the supernovae explosion. The
explosion just pushed that material outwards.
That's my stellar nucleosynthesis lesson.
Hope it was helpful,
Amara
********************************************************************
Amara Graps email: amara@amara.com
Computational Physics vita: finger agraps@shell5.ba.best.com
Multiplex Answers URL: http://www.amara.com/
********************************************************************
"Trust in the Universe, but tie up your camels first."
(adaptation of a Sufi proverb)
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